Currently, there are various techniques being used in the electronic component industry for depositing or plating metal (e.g., copper, cobalt, gold, and nickel) onto the surfaces of electronic components. Such methods include, for example, chemical vapor deposition, metal sputtering, electroplating, and electroless metal deposition.
Electroless metal deposition has become more popular in recent years and involves depositing metal onto the surfaces of electronic components in the absence of an electrical current (i.e., electrolessly). Examples of where electroless metal deposition has been used in the electronic assembly industry are in the deposition of copper on printed circuit boards. Also, in semiconductors, electroless deposition is used to deposit nickel on bonding packs, and in multichip modules, electroless deposition is used to deposit copper interconnects.
Electroless deposition of metal is typically carried out by first "activating" the surface of an electronic component by seeding or depositing a substance that will promote metal deposition onto the electronic component surface. However, it is possible that seeding may not be necessary. For example, on a substrate containing cobalt, nickel, rhodium, or palladium, seeding may not be necessary to promote metal deposition. Seeding, when desired, can be accomplished for example through immersing the electronic component in a solution containing a seeding agent. Following activation, the electronic component is typically immersed in a solution that contains metal ions and a reducing agent. The reducing agent provides a source of electrons for the metal ions, so that metal ions near or at the surfaces of the electronic components are reduced to metal and plated out onto the electronic components.
There are various metals that can be deposited electrolessly onto an electronic component including for example copper, nickel, cobalt, gold, silver, palladium, platinum, rhodium, iron, aluminum, tantalum, titanium nitride, titanium, tungsten, tantalum nitride, tungsten nitride, cobalt tungsten phosphorous, or combinations thereof. A metal that has been of particular interest recently is copper.
The electroless deposition of copper is most commonly carried out by seeding or activating the surface of the electronic component by briefly immersing the electronic component in a solution containing palladium. The palladium upon contact with the electronic component will deposit on conductive metallic surfaces such as aluminum, titanium nitride, tantalum nitride, tungsten and copper, while leaving oxide containing surfaces unseeded.
Following seeding, electroless copper deposition typically includes a two-step reaction; reduction of the seeded conductive surface followed by plating of ionized copper onto the reduced conductive surface. A typical reductant for the electroless deposition of copper is formaldehyde (CH.sub.2 O). In this case, the first step is given by: EQU CH.sub.2 O+2OH.fwdarw.HCOO.sup.- +H.sub.ad +H.sub.2 O+e.sup.-(Eqn. 1)
Where H.sub.ad represents hydrogen adsorbed onto the surface. The adsorbed hydrogen can further react in one of two manners: EQU 2H.sub.ad .fwdarw.H.sub.2 (Eqn. 2) or EQU H.sub.ad +OH.sup.- .fwdarw.H.sub.2 O+e.sup.- (Eqn. 3)
It is preferable to have water and an additional electron formed (i.e., Eqn. 3), as the formation of hydrogen gas (Eqn. 2) can lead to bubbles, and subsequent uneven metallic deposition. Thus, reaction conditions favoring Equation 3, such as high pH and slow metal deposition rates are preferred. Once the surface of the electronic component is reduced, if ionic copper (copper in an oxidized state) is present near or at the surface, it will plate out on the surface (Cu.sup.++ /Cu.sup.+ is reduced to Cu.sup.0 on conductive surfaces which supply the electrons): EQU Cu.sup.++ +2e.sup.- .fwdarw.Cu.sup.0 (Eqn. 4) EQU Cu.sup.+ +e.sup.- .fwdarw.Cu.sup.0 (Eqn. 5)
Electroless metal deposition, when performed in a wet processing system, has typically been performed in a system containing multiple open baths (e.g., a wet bench). The use of a multiple open bath system has many disadvantages. For example, oxygen has an impact on electroless metal deposition. For example, during activation, oxygen hinders seeding and can also react with the seeding agent once deposited to make the seeding agent ineffective during metal deposition. However, during the metal deposition step, oxygen can prevent degradation of the metal deposition solution, and can also slow the rate of metal deposition for better control of the process. In an open multiple bath system, it is very difficult to control oxygen levels since the baths are open to the atmosphere, and the electronic components are transferred from one bath to another. Additionally, since the bath solutions are changed infrequently, batch to batch variations in metal deposition are frequently encountered due to decomposition or fluctuating concentrations of reagents in the solutions.
U.S. Pat. No. 5,830,805 to Shacham-Diamand et al. (hereinafter "the '805 patent") proposes a solution to the problem of oxygen exposure found in open bath systems. The '805 patent discloses an apparatus and method of electroless deposition that includes processing wafers in a single sealed process chamber where a variety of fluids can be fed sequentially into the process chamber. Despite the advantages of using a sealed process chamber, the apparatus and method disclosed in the '805 patent also has disadvantages. For example, the '805 patent discloses minimizing the level of oxygen (e.g., in the absence of air) in all processing steps, despite the advantages of oxygen in certain steps in electroless metal deposition.
Additionally, the apparatus and method disclosed in the '805 patent reuses and recirculates its solutions. However, research has shown, for example, that fresh activator solution leads to significantly improved metal deposition rates in comparison to re-used activator solution. See R. Palmans, K. Maex, Feasibility Study of Electroless Copper Deposition for VLSI, 53 Applied Surface Science (1991), pp.345-352, incorporated herein by reference. Additionally, changes in concentration of solutions through reuse can lead to inconsistent processing results as with multiple open bath systems.
The present invention seeks overcome these problems, for example, by providing methods of electroless metal deposition where the activation solution and metal depositions are preferably used one time, without reuse. The present invention also provides, for example, methods of controlling oxygen levels in the solutions during electroless metal deposition based on the type of solution being contacted with the electronic components.